A transmission described in Reed U.S. Pat. No. 3,815,698 and subsequent disclosures is commercially recognized as the HMPT500 hydromechanical crossdrive steering transmission, and is utilized on several military tracked vehicles such as the US Army Bradley Fighting Vehicle. The disclosure of the Reed patent is incorporated herein by reference. This highly successful transmission has been in continuous production since 1972. Some of the unique attributes of this transmission are its infinite ratio, stepless shift capability over three ratio ranges; as well as the utilization of two large capacity, bi-directional, ball-piston hydrostatic units. These units also provide a fully integrated steer capability under all operating conditions. A single reverse ratio range is also provided. Later improvements to this transmission include an extended ratio third range feature, and an electronic control system. However, increased vehicle performance requirements have shown that additional improvements to this transmission are necessary.
Increased vehicle armor has nearly doubled the original weight of many military tracked vehicles, and has become a factor in new vehicle design. With this higher weight has come the need for greater transmission output torque and increased engine horsepower to maintain adequate vehicle performance. These two requirements have severely challenged the capacity of the HMPT500 transmission, and have necessitated the improvements disclosed by this invention.
It would be desirable to restore the performance of the transmission by generating higher output torque by adopting a unique gearing architecture, while reducing the load levels of existing components in order to accommodate higher power levels.
It would also be desirable to improve overall transmission efficiency by reducing the magnitude of regenerative power losses in the higher ratio ranges; as well as improve shift performance to achieve a better “power match” at the shift points. This “power match” feature is described in the present inventor's U.S. Pat. No. 5,730,678, the disclosure of which is incorporated herein by reference.